Multi-station transmission method and receiver for inverse transforming two pseudo-orthogonal transmission sequences used for metric calculation and base station selection based thereon

ABSTRACT

In a multi-station transmission method and receiver using training signals, a forward signal is transformed in a signal transformation part to two pseudo-orthogonal transmission signal sequences, which are framed in base stations of two adjacent zones and augmented with orthogonal training signals, thereafter being transmitted over the same channels. A signal received by a receiver of a mobile station is separated, by a signal separation part using the training signals corresponding to the respective base stations, into signal sequences received from the respective base stations. The received signal sequences are subjected to an inverse transformation by inverse transformation circuits to obtain two transmitted signal sequences, and one of these signal sequences which has a larger metric is selectively outputted.

TECHNICAL FIELD

The present invention relates to a multi-station transmission methodwhich is used in a mobile communication system and a broadcasting systemto transmit the same signal from a plurality of stations with a view toholding the continuity of a signal in the vicinity of the one boundary,and the invention also pertains to a receiver therefor.

In radio communication, especially in mobile communication, it isnecessary to implement many channels in a limited frequency is animportant technique For example, a cellular system has been employed inmobile communication, in the cellular system, the service area isdivided into plurality of cells, i.e., zones, and different frequenciesare assigned to the cells to prevent interference between them, but in abid to promote the spatial reuse of frequency, it is customary to assignthe same frequency to cells that are far enough apart not to interferewith each other. Such a cellular system requires a handoff capabilitywhich enables the mobile station to keep up conversation when it movesfrom one cell to another, that is, from one zone to another.

FIG. 1 shows the principles of a conventional zone switching scheme. Letit be assured that zones Z1 and Z2 covered by base stations BS1 and BS2are adjacent and that a mobile station M is now moving across theboundary of the zones Z1 and Z2 in a direction from the base station BS1toward the base station BS2. Forward signals to be sent from the basestations BS1 and BS2 to the mobile station M are transmitted from aswitching center 13 to the two base stations BS1 and BS2 which areswitched from the one to the other. A forward radio channel to themobile station M is set first by a first channel CH1 via the basestation BS1. When the field intensity of the first channel CH1 decreaseswith the movement of the mobile station M, a second channel CH2 is setas the forward radio channel via the base station BS2, while at the sametime the first channel CH1 is cut off. Since an access channel isusually set up by a FDMA (Frequency Division Multiple Access) or TDMA(Time Division Multiple Access) scheme, the same channel cannot be usedin adjacent zones. Hence, the two channels CH1 and CH2 use differentcarrier frequencies. On this account, the channels cannot continuouslybe switched from one to the other, inevitably resulting in a momentaryinterruption during switching. In voice communication this interruptioncan be made sufficiently short as not to seriously affect the speechquality, but in multimedia transmission such as visual or datacommunication the momentary interruption causes significant qualitydeterioration because of high-speed transmission of digital signals inmany cases.

On the other hand, in zone switching by a CDMA (Code Division MultipleAccess) scheme, a spreading code is used for channel setting and thesame carrier frequency is used in common to adjacent zones. Then when amulti-station transmission is carried out using different spreadingcodes for the adjacent zones, signals from two base stations can easilybe received and interruption-free reception is possible. However, thismethod is inherent to the CDMA scheme and cannot be applied to the FDMAand TDMA schemes. Furthermore, to identify the respective channeldefined by the spreading code, it is necessary to use a differentspreading code for each channel. There is another method which, insteadof changing the spreading code, shifts its timing to avoid overlappingof pulses detected by the two inverse transformation circuits fordespreading of the two base station is, but highly accurate transmissiontiming must be provided between the base stations.

On the other hand, a forward control signal in the mobile communicationsystem needs to call up mobile stations over a wide area. To cover awide area with a low transmission output, a multi-station transmissionsystem is needed which divides the area into a plurality of zones andtransmits the same signal to the respective zones.

In the multi-station transmission system, even if the same signals aretransmitted from the respective zones, they do not completely match interms of transmission carrier frequency and transmission data timing,posing a problem that the signals from the plurality of zones interferewith each other fit the boundary between them. To solve this problem,frequency offset type transmitter diversity or the like has beenemployed. This method is one that offsets the transmitter carrierfrequency of each zone in the range of from ½ to ¼ of the modulationband and receives the frequency offset signals by a differentialdetector at the receiving side, thus enabling a diversity, reception.However, this method has a disadvantage in that if the data timing isnot the same, interference will occur and the frequency offsettingenlarges the receiving band width correspondingly, making it hard toimplement a narrow-band communication

An object of the present invention is to provide a multistationtransmission method and a receiver therefor which, regardless of theaccess scheme used, allow zone switching free from signal discontinuityand enable simultaneous reception of identical signals from a 1plurality of base stations without widening the receiving band, therebyimplementing highly reliable reception based on the diversity effect.

SUMMARY OF THE INVENTION

In a mobile communication system in which the service area is broken upinto a plurality of zones each having a base station and a mobilestation performs communication via the base station of its visited zone,the multi-station transmission method according to the presentinvention, transmits the same signal from the base station of thevisited zone and the base station of at least one adjacent zone when themobile station moves across the boundary between its visited zone andthe adjacent zone, the method comprising the following steps

(a) the same forward signal sequence destined to the mobile station istransmitted to N base stations including the base station of the mobilestation's visited zone and the base station of at least one adjacentzone, N being an integer equal to or greater than 2;

(b) each of the N base stations each converts the forward signalsequence to a transmission signal sequence and adds predeterminedpseudo-orthogonal training signals to the transmission signal sequencefor each frame to generate a framed signal sequence;

(c) the N base stations each send the framed signal sequence by atransmission radio wave of the same channel; and

(d) said mobile station receives the transmitted radio wave from eachbase station, then separates It into N transmitted signal sequences fromthe N base stations through utilization of the previously known Ntraining signals and obtains a desired received signal sequence from thetransmitted signal sequences.

The receiver according to the present invention, is provided with:separating means which separates received waves of the same channel bytheir training signals into a plurality of signal sequencescorresponding to the training signals, respectively; inversetransformation means which subject these separated signal sequences totransformation inverse from that effected thereon at the transmittingsides to restore the original signal sequences; and means which outputsthat one of the restored signal sequences which is high in reliabilityat the time of separation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the principles of a conventional zoneswitching scheme;

FIG. 2 is a block diagram illustrating the transmitting side in anembodiment of the present invention;

FIG. 3A is a diagram showing the construction of a signal transformationpart 14 in FIG. 2 when it is implemented by an interleave scheme usingmemories;

FIG. 3B is a diagram showing the construction of the signaltransformation part 14 when it is implemented by a scramble scheme;

FIG. 4 is a diagram showing the frame configuration of a transmissionsignal;

FIG. 5 is a block diagram of the receiving side in an embodiment of thepresent invention;

FIG. 6A in a block diagram of a nonlinear interference canceller formingthe principal part of a signal separation part; and

FIG. 6B is a block diagram illustrating a linear interference canceller.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 2 there is illustrated the base station side of an embodimentaccording to the present invention At the base station side, a forwardinput signal DI destined for a mobile station M is input into a signaltransformation part 14 from a switching center 13. The forward signal DIis transformed by the signal transformation part 14 into two differenttransmission signal sequences T1 and T2. This transformation is intendedto enable the two transmission signal sequences T1 and T2 to be regardedas statistically independent (i.e., orthogonal or pseudo-orthogonal)signal sequences which have substantially zero cross-correlation of ±10symbols or so this is done by interleaves of different sizes, example.The transformation by interleaving is performed, as schematically shownin FIG. 3A, by writing the signal sequence DI into memories 14M1 and14M2 of different sizes in the row direction as indicated by the brokenline arrows and then reading out the memories 14M1 and 14M2 in thecolumn direction as indicated by the full-line arrows. By using memoriesof the same size but different in the length-to-width ratio, theinterleaved signal sequences T1 and T2 become pseudo-orthogonal.Moreover, even if a burst-like error is induced on the transmissionline, it is dispersed when the original signal sequence is restored byinverse transformation; hence, error correction is effectively made.Alternatively, the input signal DI may be divided into two transmissionsignal sequences T1 and T2 one of which is interleaved but the other ofwhich is not.

Another method of generating signal sequence of substantially zerocross-correlation is shown in FIG. 3B in which the transmission signalsequences T1 and T2 are generated by scrambling the input signal DI inscramblers 14S1 and 14S2 with different scramble codes SC1 and SC2 fromscramble code generating parts 14C1 and 14C2. Also in this case, onlyone of the transmission signal sequences T1 and T2 may be formed by ascrambled version of the input signal DI. Incidentally, the signaltransformation part 14 may be provided In the switching center 13, orits respective components may be provided in the corresponding basestations BS1 and BS2, or it may be provided singly. According to thepresent invention, as referred to later on, the training signals thatare added to each frame in the base stations BS1 and BS2 need only to beat least orthogonal to each other and the signal transformation part 14may be omitted. In such an instance, the input signal DI is applied toframing circuits FR1 and FR2 of the base stations BS1 and BS2. Thesignal transformation part 14 may add an error correcting code to eachtransmission signal sequence, if necessary.

These transmission signal sequences T1 and T2 are sent to the bassstations BS1 and BS2 in adjacent zones Z1 and Z2, respectively. In thebase stations BS1 and BS2, the transmission signal sequences T1 and T2are converted by framing circuits FR1 and FR2 to framed signal sequencesF1 and F2, respectively. In the present invention, as shown in FIG. 4,different and inherent training signals TRN1 and TRN2, which are held inregisters RG1 and RG2 in the base stations BS1 and BS2, are added totransmission data DAtA (the transmission signal sequences T1 and T2) ofa fixed length to form respective frames. The training signals TRN1 andTRN2 used are those which are orthogonal or pseudo-orthogonal to eachother, that is, signals such that the sum of multiplied values ofcorresponding symbols of the training signals TEN1 and TRN2 issubstantially zero.

By using such signals of excellent orthogonality as the training signalsTRN1 and TRN2, it is possible to increase the accuracy of coefficientsetting (setting of a tap coefficient or setting of a weightcoefficient) which is accompanied by correlation processing in a signalseparation part 20 In FIG. 5 described later on. In the case whereinter-symbol interference occurs owing to a delay time dispersion In amulti-path propagation path, however, equalization processing isinvolved in the signal separation part 20, hence respective multi-pathcomponents must be separated. To meet this requirement, the trainingsignals TRN1 and TRN2 need to be excellent in auto-correlationcharacteristic as well as in orthogonality. That is, the auto correctionfunction of each of the training signals TRN1 and TRN2 is preferably afunction which has a sharp peak at a position corresponding to a timedifference 0 (a phase difference 0) but becomes negligibly small inother regions. The training signals of excellent auto-correction can beused as signals for timing regeneration. Since the above-said twocharacteristics, that is, the orthogonality and the auto-correctionproperty, influence each other, it is desirable to optimize them in thesystem employing the present invention.

The framed signal sequences F1 and F2 generated by the framed circuitsFR1 and FR2 in FIG. 2 are converted by transmitters TR1 and TR2 tomodulated waves C1 and C2, respectively, which are transmitted viatransmitting antennas ANT-T1 and ANT-T2. Let it be assumed that the twomodulated waves C1 and C2 use the same channel selected from a channelgroup of FDMA, TDMA and CDMA schemes. Furthermore, suppose that thetransmitting antennas ANT-T1 and ANT-T2 are so distant from each otherthat when the two modulated waves C1 and C2 are received by the mobilestation M, their variations by radio wave propagation can besufficiently independent of each other.

FIG. 5 is a block diagram illustrating the construction of each mobilestation M. The mobile station M simultaneously receives the modulatedwaves C1 and C2 of the same channel as their combined wave by used of areceiving antenna ANT-R. The received signal is demodulated by areceiver 19R and the resulting base band signal is outputted therefromas a digital signal. The base band signal is applied to a signalseparation part 20 wherein it is amplified and then separated intoreceived signal sequences R1 and R2 corresponding to the modulated wavesC1 and C2. This separation uses the training signals contained in eachof the modulated waves, and the separation can be carried out using thetechnique of what is called an interference canceller. The interferencecanceller schemes can be classified into a nonlinear interferencecanceller and a linear interference canceller. When only one receivingantenna ANT-R is used as in the case of FIG. 5, only the nonlinearinterference canceller is applicable. When two or more receivingantennas are used, either of the nonlinear and linear interferencecancellers can be used.

The operation of the linear interference canceller is disclosed indetail in R. T. -Compton, Jr., “Adaptive Antennas, Concept andPerformance”, Prentice-Hall, 1988 or Suzuki, “Signal TransmissionCharacteristics In Least Square Combining Diversity Reception”, Journalof the Institute of Electronics, Information and Communication Engineersof Japan, B-II, vol. J75-B-II, No. 8, pp. 524-534, August, 1992: theoperation of the nonlinear interference canceller is described in detailin Hitoshi Yoshino and Hiroshi Suzuki, “Adaptive Interference CancellerExtended from RLS-MLSE”, Technical Report of the Institute ofElectronics, Information and Communication Engineers of Japan, TechnicalReport RCS92-120 (1993-01). In either case, received signals from apredetermined number of base stations which can be predicted areseparated into individual received signals, the received signals excepta noted desired received signal are regarded as interference signals,and these separated interference signals are subtracted from thereceived signals of the combined received wave, by which the backgroundnoise of the desired received signal is remarkably reduced. The presentinvention separates all the received signals by similar processing,regarding the individual received signals as desired received signals.

The separated received signal sequences R1 and R2 are provided toinverse transformation circuits 31 and 32, wherein they are subjected tothe inverse transformation processing shown in FIGS. 3A or 3B, by whichtransmitted signal sequences SR1 and SR2 are obtained as received signalsequences. The received signal sequences SR1 and SR2 are fed to a signalreconstruction part 33, which selects one of the received signalsequences on the basis of likelihood values M1′ and M2′ correrponding toestimated errors obtained in the signal separation processing in thesignal separation part 20 and outputs the selected signal sequence to anoutput terminal OUT.

Next, a description will be given of an example of the basicconfiguration of the interference canceller in the signal separationpart 20. FIG. 6A is a block diagram of the non-linear interferencecanceller and FIG. 6B is a block diagram of the linear interferencecanceller. In the nonlinear interference canceller of FIG. 6A, a samplevalue Y(n) of the base band signal, obtained by the detection of thecombined wave of the two modulated waves C1 and C2 by the receiver 19R,is provided as an input signal to an input terminal 2T. On the otherhand, upon each application of the input signal Y(n) to the is inputterminal 2T, a maximum likelihood sequence estimator 24 generates twosignal sequence candidates (code sequence candidates) CSC1 and CSC2 eachhaving a predetermined number of states for signal transition andprovides them to replica generators 22R1 and 22R2. The replicagenerators 22R1 and 22R2 are formed by transversal filters to whichparameters for estimating the channel characteristics of the modulatedwaves C1 and C2, that is,.impulse responses H1 and H2 of respectivechannels, are provided as tap coefficients; the replica generatorsgenerate estimated signals or replicas RP1 and RP2 by inner productcalculations (convoluting calculations) of the signal sequencecandidates CSC1 and CSC2 and the tap coefficients H1 and H2.

These replicas RP1 and RP2 are provided to subtracters 21A1 and 21A2,wherein they are subtracted from the input signal Y(n) to obtain anestimation error ε: this processing is repeated for all candidates ofthe two signal sequences. As a result, two code sequence candidates, forwhich the square |ε|² of the estimation error available for a likelihoodcalculation pert 23, are determined an two most likely code sequencesand estimated transmitted signals R1 and R2 are provided to outputterminals on the basis of such code sequences. At the same time, metricsM1 and N2 of the code sequences are calculated from the estimation errorε and are outputted. The maximum likelihood sequence estimation methodto described In the aforementioned literatures and is disclosed indetail in PCT Application Publication WO94/17600 (published Aug. 4,1994) as well. For example, the Viterbi algorithm way be used as one ofthe maximum likelihood sequence estimation algorithms.

The mobile station M (FIG. 2) holds in registers. 27G1 and 27G2 trainingsignal patterns TRN1 and TRN2 of the visited zone Z1 and the adjacentzone Z2 received from the base station BS1 via a control channel.Alternatively, the mobile station M prestores, as a table in a memory,the training signal patterns TRN1, RN2, . . . corresponding toidentification numbers assigned to the zones Z1, Z2, . . . , in whichcase the training signal patterns are read out from the table by use ofthe identification numbers of the zone Z1 and the adjacent zone Z2received via the control channel from the base station of the visitedzone Z1 and are set in the registers 27G1 and 27G2. During the receptionof the training signals TRN1 and TRN2 in each frame by the receiver 19Rof the mobile station M, the respective training signal patterns TRN1and TRN2 are provided from the registers 27G1 and 27G2 to the channelparameter estimation part 25 and the replica generators 22R1 and 22R2via switches 26S1 and 26S2.

The replica generators 22R1 and 22R2 are controlled by the tapcoefficients H1 and H2 provided thereto to generate replicas (estimatedreceived signal training signals) of the received signals from thetraining signal patterns TRN1 and TRN2 and provide the replicas to thesubtractors 21A1 and 21A2. The parameter estimation part 25 determines,for example, by an adaptive algorithm, the tap coefficients H1 and H2for the training patterns TRN1 and TRN2 in such a manner as to minimizethe power |ε|² of the estimation error signal. The replica generators(transversal filters) 22R1 and 22R2, supplied with such tap coefficientsH1 and H2, are regarded as simulating the characteristic (impulseresponse) of the channels over which the modulated signals C1 and C2propagate, respectively. During the reception of the data DATA in thereceived frame, the tap coefficients H1 and H2 determined as mentionedabove are provided to the replica generators 22R1 and 22R2 and themaximum likelihood sequence estimator 24 makes a maximum estimation of apair of transmitted signal sequences (transmitted data) as describedpreviously. Furthermore, the maximum likelihood sequence estimatorcalculates and outputs the metrics (the reliability of the estimatedsignal sequences) M1 and M2 of the decision paths from the likelihood(1/|ε|², for example) used for the decision of the signal sequences R1and R2 by a known method. When the input signal sequence is transformedin the signal transformation part 14 to the transmission signalsequences T1 and T2 which are pseudo-orthogonal to each other asdepicted in FIG. 2, the tap coefficients H1 and H2 can be corrected, asrequired, in the above-described fashion to minimize the estimationerror power |ε|² again through utilization of the two decidedtransmitted signal sequences during the data DATA receiving period. Inthe example of FIG. 6A, the metrics M1 and M2 are the same value. Whilein the above the operation by a single branch has been described, theconfiguration of diversity reception is also possible, in which case,too, the interference canceller similarly operates.

FIG. 6B shown the case where the signal separation part 20 is formed bythe linear interference canceller. In this instance, combined receivedwaves received by two receiving antennas ANT-R1 and ANT-R2 are convertedby receivers 19R1 and 19R2 to base band signals Y1 and Y2, respectively,which are applied to input terminals 2T1 and 2T2 of the signalseparation part 20. These base band signals Y1 and Y2 are weighted withweighting factors W₁₁ and W₁₂ in weighting circuits 21W₁₁ and 21W₁₂,respectively, and are added together in an adder circuit 22A1, theoutput of which is provided as an estimated signal for the onetransmitted modulated signal C1. The estimated signal output is fed to adecision circuit 24D1, wherein it is decided to be larger or smallerthan a threshold value and from which it is provided as the transmissionsignal sequence R1 to an output terminal. The difference (an estimationerror) between the input and the output signal of the decision circuit24D1 is detected by a difference circuit 23E1 and is outputted as themetric signal M1.

During the period of receiving the training signals in the transmittedframe, the training signal pattern TRN1 is provided, as a substitute forthe decided output, to the difference circuit 23E1 from the register27G1 via the switch, and a control circuit 25C1 determines the weightingfactors W₁₁ and W₁₂ in such a manner as to minimize the square |ε| ofthe absolute value of the difference. The thus determined factors W₁₁,and W₁₂ are used to perform a weighted addition of the received signalsY1 and Y2 during the period of receiving the data in the transmittedframe, by which the estimated transmitted signal R1 can be obtained. Thereason for which the difference output from the difference circuit 23E1,that is, the error component ε, becomes small is that the modulated waveC2 is cancelled.

Similarly, the signals Y1 and Y2 from the input terminals 2T1 and 2T2are weighted by weighting circuits 21W₂₁ and 21W₂₂, respectively, andare added together by an adder circuit 22A2, and the added output issubjected to a level decision by a decision circuit 24D2. In thetraining signal receiving period the training signal pattern TRN2 fromthe register 27G2 is provided via the switch 26S2 to a differencecircuit 23E2, by which the difference between the training signalpattern and the output from the adder circuit 22A2 is obtained. Theweighting factors W₂₁ and W₂₂ are determined by a control circuit 25C2so that the difference becomes minimum. By performing a weightedaddition of the input signals Y1 and Y2 through use of such weightingfactors during the period of receiving the data DATA in the receivedframe, the modulated wave C1 is cancelled and the transmitted signalsequence R2 is outputted. In the example of FIG. 6B the metric signalsM1 and M2 differ from each other. It is also possible that the sum ofsquares of the two metric signals M1 and M2 is distributed as a commonmetric signal as in the FIG. 6A example.

Thus, during the reception of the training signals the tap coefficientsH1 and H2 are correctly set by the channel parameter estimation part 25in FIG. 6A, or in FIG. 6B the weighting factors W₁₁, W₁₂, W₂₁ and W₂₂are correctly determined.

In the interference cancellers of FIGS. 6A and 6B, when the onemodulated wave, for example C1, is extracted, the other modulated waveC2 is handled as an interference wave-by this, the demodulatedtransmitted signal sequences R1 and R2 corresponding to the transmissionsignal sequences T1 and T2 contained in the respective modulated waveare extracted. The thus extracted transmitted signal sequences R1 and R2are provided to the inverse transformation circuits 31 and 32, whereinthey are subjected to a transformation inverse from that in the signaltransformation part 14 (FIG. 2) at the transmitting side: thus, thereceived signal sequences SR1 and SR2 are generated. When the signaltransformation part 14 at the transmitting side carries out suchinterleave as shown in FIG. 3A, two memories of different sizes, similarto those in FIG. 3A, are provided in the inverse transformation circuits31 and 32, respectively, and are configured so that the received signalsequences R1 and R2 are read in the column direction and read out in therow direction, just opposite in direction from that in FIG. 3A. When thetransmitting side effects the signal transformation by the scramblecodes SC1 and SC2 as shown in FIG. 3B, multipliers are provided in theinverse transformation circuits 31 and 32 to descramble the receivedsignal sequences R1 and R2 with the scramble codes SC1 and SC2.

The metric signals M1 and M2 representing the reliability of the signalsequences at the time of their separation, which are provided from theinterference canceller in the signal separation part 20, are outputtedin synchronization with the signal sequences SR1 and SR2. The metric isexpressed by the level of the inverse 1/|ε| of the estimation error inthe separation processing or its square or negative −|ε| or −|ε|², thelarger the value, the higher the reliability. Moreover, in the inversetransformation, general metric values M1′ and M2′ of the received signalsequences SR1 and SR2 are generated using the metric used for errorcorrection decoding. If the transformation in the signal transformationpart 14 in a mere reversal of the order of the interleave or the like,the metric signals M1′ and M2′ by the inverse transformation are signalswhich are merely reverse in order from the metric signals M1 and M2. Thetwo received signal sequences SR1 and SR2 of different metrics areprovided from the inverse transformation circuits 31 and 32 to thesignal reconstruction part 33, which generates optimal demodulated dataDO and provides it to the output terminal OUT. The demodulated data canbe generated by various methods such as those (1) which selects thereceived signal sequence of the larger metric, (2) which weights thedecided received signal sequence with the metric, then combines it withthe other signal sequence and makes a decision, and (3) which performsonly interleave in the inverse transformation and performs errorcorrection decoding while selecting data of the received signal sequenceof the larger metric.

The operation described above is basically the same in the handoff andthe multi-station transmission system. However, the handoff has acapability of stopping signal transmission from the old zone when theintensity of the field for receiving radio waves from the new zoneincreases. While in the above description the same signal has been sentfrom two base stations, it may be sent from three or more base stationsBS1, BS2, BS3, . . . as indicated by the broken-lined base station BS3of a third adjacent zone in FIG. 2. Letting the number of base stationsbe represented by N, the signal separation part 20 in FIG. 6A needs onlyto be provided with N subtraction circuits 21A1, 21A2, . . . , N replicagenerators 22R1, 22R2, . . . , N switches 2681, 2682, . . . , and Nregisters 27G1, 27G2, . . . . In the case of the signal separation part20 shown in FIG. 6B, N combiners 22A1, 22A2, . . . , N decision circuits24D1, 24D2, . . . , N difference circuits 23E1, 23E2, . . . , N controlcircuits 25C1, 25C2, . . . , N switches 26S1, 26S2, . . . , and Nregisters 27G1, 27G2, . . . , are provided In association with thesignals Y1, Y2, . . . , from N receivers 19R1, 19R2, . . . .Furthermore, N² weighting circuit 21W₁₁, . . . , 21W_(KK) are providedfor conducting N sots of weighted additions for the N input signals Y1,Y2, . . . .

Thus, the present invention permits zone switching through use of thesame channel regardless of the access scheme used. Since no interruptionoccurs at the time of zone switching, the reliability of fast digitalsignal transmission will not be Impaired. Besides, the receiving fieldintensity decreases at the time of zone switching, since the mobilestation usually moves near the zone boundary; according to the presentInvention, however, the mobile station simultaneously receives signalsfrom a plurality of base stations—this produces a diversity effect andhence improves the transmission characteristic.

Hence, the present invention Is effective when applied to high-capacity,multimedia-oriented digital mobile communications and portable telephonesystems, furthermore, it is effective when dividing a wide area into aplurality of zones and performing transmission in the broadcast mode.

What is claimed is:
 1. A receiver for a mobile station which receives,from a base station in each of N adjacent zones, a modulated waveobtained by framing N pseudo-orthogonal signal sequences resulting froma transformation of a same signal and added with orthogonal trainingsignals and selectively outputs the signal sequence transmitted from adesired base station, N being an integer equal to or greater than 2,comprising: N antennas (ANT-R1, ANT-R2) provided at different positionsfrom one another for receiving transmitted waves from base stations andproducing a combined wave signal of the transmitted waves; N radiofrequency receivers (19R1, 19R2) each for receiving the combined wavesignal from corresponding one of said N antennas and producing areceived signal; and N weighted addition means 21W₁₁, 21W₁₂, 21W₂₁,21W₂₂, 22A1, 22A2) for generating N weighted added values for N receivedsignals from said N antennas; N decision means 24D1, 24D2) fordetermining levels of outputs from said N weighted addition means andoutputting said received signals; pattern holding means (27G1, 27G2) forholding training signal patterns peculiar to said N adjacent zones; Nsubtracting means (23E1, 23E2) each for obtaining the difference betweenthe output from one of said weighted addition means and said trainingsignal patterns from one of said pattern holding means during thereception of said training signal in said received signal; and N controlmeans (25C1, 25C2) for determining a weighting factor of said N weightedaddition means which minimizes said difference, during the trainingsignal receiving period.
 2. A multi-station transmission method for usein a mobile communications system in which a service area is dividedinto a plurality of zones each having a base station, a mobile stationperforms communication via said base station of its visited zone, and asame signal sequence is transmitted from both said base station of saidvisited zone and the base station of at least one adjacent zone whensaid mobile station moves across the boundary between said visited zoneand said adjacent zone, said method comprising the following steps: (a)transmitting a forward signal sequence (DI) destined for said mobilestation to N said base stations (BS1, BS2) including said base stationof said visited zone and said base station of said at least one adjacentzone, N being an integer equal to or greater than 2; (b) rendering fromeach of said N base stations said forward signal sequence to atransmission signal sequence (T1) and adding predetermined one oftraining signals (TRN1, TRN2), which are pseudo-orthogonal to oneanother, to said transmission signal sequence for each frame to generatea framed signal sequence (F1, F2) having a training signal period (TRN1,TRN2) and a data signal period (DATA) corresponding to the trainingsignal and the transmission signal sequence, respectively; (c)transmitting from each of said N base stations said framed signalsequence on a transmission radio wave (C1, C2) of a same channel; (d)receiving by said mobile station transmission radio waves from said Nbase stations, and estimating characteristics of channels from said Nbase stations through use of said N preknown training signals (TRN1,TRN2) during the reception of said training signals in said receivedsignals; (e) generating N replicas (RP1, RP2) which simulate said framedsignal sequences (F1, F2), from said estimated channel characteristics;(f) subtracting said N replicas from said received signal sequences toproduce an estimation error (ε); (g) performing a maximum likelihoodestimation in a manner to minimize the resulting estimation error (ε),thereby determining said N transmitted signal sequences; and (h)selectively delivering one of said n transmission signal sequences (DO).3. A multi-station transmission method for use in a mobile communicationsystem in which a service area is divided into a plurality of zones eachhaving a base station, a mobile station performs communication via saidbase station of its visited zone, and a same signal sequence istransmitted from both said base station of said visited zone and thebase station of at least one adjacent zone when said mobile stationmoves across the boundary between said visited zone and said adjacentzone, said method comprising the following steps: (a) transmitting aforward signal sequence (DI) destined for said mobile station to N basestations (BS1, BS2) including said base station of said visited zone andsaid base station of said at least one adjacent zone, N being an integerequal to or greater than 2; (b) rendering from each of said N basestations said forward signal sequence to a transmission signal sequence(T1, T2) and adding predetermined one of training signals (TRN1, TRN2),which are pseudo-orthogonal to one another, to said transmission signalsequence for each frame to generate a framed signal sequence (F1, F2)having a training signal period (TRN1, TRN2) and a data signal period(DATA) corresponding to the training signal and the transmission signalsequence, respectively; (c) transmitting from each of said N basestations said framed signal sequence (F1, F2) on a transmission radiowave (C1, C2) of a same channel; (d) receiving said mobile station thecombined wave of said transmission radio waves by N receivers (19R1,19R2) via N different antennas (ANT-R1 ANT-R2) to produce N base bandoutputs (Y1, Y2); (e) determining N sets of weighting factors (W₁₁, W₁₂,W₂₁, W₂₂), each set composed of N weighting factors, by determining theN weighting factors for each of said N training signals (TRN1, TRN2), sothat an estimated signal is obtained by weighting said N base bandoutputs (Y1, Y2) from said N receivers with the N weighting factors toproduce N weighted base band outputs and adding (22A1, 22A2) the Nweighted base band outputs together during the training signal period,matches (23E1, 23E2) one of said N training signals (TRN1, TRN2); (f)obtaining N output signals (R1, R2), by weighting said N base bandoutputs (Y1, Y2) with N weighting factors of each of said N sets andadding the weighted N base band outputs together during the data signalperiod, as said N transmission signal sequences; and (g) selectivelydelivering one of said N transmission signal sequences.
 4. A receiverfor a mobile station which receives, from a base station in each Nadjacent zones, a modulated wave obtained by framing N pseudo-orthogonalsignal sequences resulting from a transformation of a same signal andaugmented by orthogonal training signals and selectively outputs thesignal sequence transmitted from a desired base stations, and receivercomprising: signal separation means (20) which separates, by N preknownpseudo-orthogonal training signal patterns, a received wave in a samechannel into N received signal sequences corresponding to said trainingsignal patterns and outputs metrics each indicating reliability of acorresponding one of said N received signal sequences, N being aninteger equal to or greater than 2; inverse transformation means (31,32) whereby said N separated received signal sequences are subjected toa transformation inverse from that effected at the transmitting side torestore said N transmitted signal sequences; and means (33) whichselectively outputs one of said N transmitted signal sequences which hasthe largest metric at the time said received wave is separated into saidN received signal sequences; wherein said signal separation means (20)comprises: N replica generating means (22R1, 22R2) which, upon eachinput of a received signal, generates N signal sequence candidates tosimulate channels from said base stations under control of given channelparameters and generates N replicas for said received signal from saidreceived signal sequence candidates; subtracting means (21A1, 21A2) forsubtracting said N replicas from said received signal and for outputtingan estimation error; maximum likelihood sequence estimation means (23,24) which, upon each input of said received signal, sequentiallygenerates all received signal sequence candidates, calculates thelikelihood for each of said candidates from said estimation error anddetermines which one of said received signal sequences has maximumlikelihood; pattern holding means (27G1, 27G2) for holding trainingsignal patterns peculiar to said N adjacent zones; switching means(26S1, 26S2) through which, during the reception of said trainingsignals in said received signal, said N training signal patterns aresupplied from said pattern holding means to said N replica generatingmeans to generate replicas of said training signals and during thereception of the other signal than said training signals in saidreceived signal, said candidates being supplied from said maximumlikelihood sequence estimation means to said N replica generating means;and channel parameter generating means (25) which generates channelparameters to be provided to said replica generating means so that saidestimation error become minimum.
 5. The method of claim 2,3,4, whichincludes, prior signal sequence into at least N different transmissionsignal sequences by a desired transformation procedure, and a step ofobtaining N received signal sequences by subjecting said N transmittedsignal sequences separated in said step (d) to processing of atransformation procedure inverse from said transformation procedure. 6.The method of claim 5, wherein said transformation procedure is one thatreduces correlation among said N transmission signal sequences.
 7. Themethod of claim 6, wherein, said transformation procedure is a procedureof interleaving said forward signal sequence at least (N-1) timings toobtain said at least N different transmission signal sequences.
 8. Themethod of claim 2, wherein said step includes a step wherein a metricfor each of said N signal sequences is calculated (24) by using said Nreplicas and said base band output and one of the sets of N signalsequence candidates (CSC1, CSC2) corresponding to a set of said Nreplicas which provided the largest metric is determined as the Ntransmission signal sequences (R1, R2).